Magnesium is one of the metal elements having the largest reserves on earth. Magnesium has excellent mechanical, physical and chemical properties and is used in various fields. In the periodic table of the elements, magnesium and lithium are located at positions diagonal from each other and these elements have the similar ionic radii and similar chemical properties. The electric potential for magnesium is higher than that of lithium (the potential for lithium is −3.03 V while the potentials for magnesium are −2.37 V (acidic) and −2.69 V (alkaline). Theoretical specific capacity of magnesium is lower than that of lithium (the theoretical specific capacity of lithium is 3862 mAh/g while that of magnesium is 2205 mAh/g). However, since magnesium is low cost, easy to handle and highly safe, a magnesium secondary battery including magnesium as a negative electrode has attracted attention as a subject of research for new types of battery systems. See Gregory T D, Hoffman R J, Winterton, Development of an ambient secondary magnesium battery, J. Electrochem. Soc., 137(1990)775-780; Aurbach D, Lu Z, Schechter A, Gofer Y, Gizbar H, Turgemann R, Cohen Y, Moshkovich M, Levi E, Prototype systems for rechargeable magnesium batteries Nature, 407(2000)724-727; YUAN Hua-tang; WU Feng; WU Xu-li; Li Qiang, The study and development of rechargeable magnesium battery, battery, 2002, 32(6): 14-17; FENG Zhen-Zhen; NULI Yan-Na; WANG Jiu-Lin; YANG Jun Development of rechargeable magnesium battery, Chemical and Physical Power Sources, 2007, 1:79-79.
An electrolytic solution for a magnesium secondary battery is closely related with reversible electrochemical deposition of magnesium. Thus, studies regarding properties of magnesium in various electrolytic solutions have been widely conducted. As an electrolytic solution having high reversibility of magnesium deposition and dissolution, a system including a Grignard reagent of an organic ether is known. However, this system of Grignard reagent has low electric conductivity (less than 0.5 ms/cm) and low anodic stability (i.e., potential of anode oxidization decomposition is less than 2.3 V vs. Mg). For example, electric conductivity of a system of ethyl magnesium bromide(chloride)/THF is as low as 0.26 ms/cm, and the potential of anode oxidization decomposition is 1.5 V vs. Mg (Genders J D, Pletcher D, Studies using microelectrodes of the Mg (II)/Mg couple in teraphydrofuran and propylene carbonate, J. Electroanal. Chem. 199(1986) 93-100; Lu Z, Schechter A, Moshkovich M, Aurbach D, On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions, J. Electroanal. Chem., 466(1999)203-217; Guo Y S, Yang J, NuLi Y N, Wang J L, Study of electronic effect of Grignard reagents on their electrochemical behavior, Electrochem. Commun., 12(2010), 1671-1673). Anodic stability of the system of a Grignard reagent-electrolytic solution is determined based on a C—Mg bond in the Grignard reagent. Since the stability of the C—Mg bond is low, when the Grignard reagent-electrolytic solution is used for an electrolytic solution for a rechargeable battery, improvement of its anodic stability, in particular, improvement of its compatibility with a material for a positive electrode is expected (Aurbach D, Moshkovich M, Schechter A, Turgeaman R, Magnesium Deposition and Dissolution Processes in Ethereal Grignard Salt Solutions Using Simultaneous EQCM-EIS and In Situ FTIR Spectroscopy, Electrochem. Solid-State Lett., 3(2000)31-34).
At present, the most advanced system for an electrolytic solution for a magnesium secondary battery is a system of 0.25 mol/L Mg(AlCl2EtBu2)/tetrahydrofuran (wherein Et is ethyl and Bu is buthyl) proposed by Israeli scientist Mr. Aurbach. The potential for anode oxidization decomposition is 2.5 V vs. Mg (Aurbach D, Luz, Schechter A, Gofer Y, Gizbar H, Turgernann R, Cohen Y, Moshkovich M, Levi E, Nature, 407(2000)724-727).
A major direction of development for current magnesium secondary batteries is to facilitate the development of magnesium secondary batteries having high performance at low cost to seek a low-cost system for an electrolytic solution having at least one of high potential of anode oxidization decomposition, high electric conductivity, high reversibility of magnesium deposition and dissolution, and excellent cycle performance.